This invention concerns a method for thermal surface treatment in a continuous casting machine and also concerns a device suitable to carry out this method. The method and relative device according to the invention are employed in the metallurgical field and in particular on continuous casting machines to perform continuous controlled cooling of the bloom being fed.
The invention is applied to the treatment of blooms, billets or slabs having a square, round, rectangular or polygonal cross-section and consisting of fine-grain structural steels.
In the description that follows, we shall use only the word "blooms" for descriptive convenience but shall mean thereby that the device is applied also to billets and slabs having any of the above cross-sections.
The invention is applied advantageously, but not only, to the treatment of the following families of steels: steels for structural uses, steels for cold processing, steels for forging, steels for cementation, hardening and tempering steels and surface hardening steels.
Moreover the invention is applied in particular to steels having an appreciable content of aluminum.
Continuous casting plants use the technique of carrying out a quenching operation on moving blooms before the blooms are fed into a furnace with a hot charge.
The quenching operation in continuous casting plants is performed downstream of the shearing assembly consisting, for instance, of a shears or oxygen-cutting torches, depending on the thickness of the bloom; this shearing assembly is installed downstream of the extraction and straightening segment of the plant.
The quenching operation, however, entails a series of drawbacks linked to the fact that the bloom arriving at the quenching station has too low a temperature.
Moreover, in the plants of the state of the art, when the quenching operation is carried out on the sheared bloom, there is a considerable difference of temperature between its two ends, and this difference of temperature results in a lack of structural homogeneity with consequent faults in the bloom and/or problems in the plant downstream.
There is also the method of cooling the blooms in air, which is advantageously a forced draught at a temperature below 600.degree. C., so as to cause conversion of the austenite in their surface layer before charging them into the furnace.
This entails in practice a cooling of the bloom, which reduces a great deal of the energy saving linked to the hot charging process.
The state of the art includes an auxiliary cooling method which employs a device arranged upstream of the extraction and straightening assembly. This method was devised essentially for carrying out thermal soft reduction, namely to reduce the segregation of the carbon on the central axis of the bloom or billet but not to reduce and eliminate the problems of hot shortness, which are characteristic of a hot charge of steels containing aluminum.
Moreover, this method is applied mainly to steels with a high carbon content and with very low percentages of aluminum.
Furthermore, in the method of the state of the art the surface temperature of the billet or bloom at the straightening assembly is about 800.degree. C. This temperature may be too high to compensate the drawing stresses arising from the extraction and straightening, for these stresses entail a possible occurrence of transverse cracks in the skin of the bar.